Thermionically-active substance and method of making the same



Aug. 7, 1923. W4,124-

W. WILSON THERMIONICALLY ACTIVE SUBSTANCE AND METHOD OF MAKING THE SAME Filed July 26. 1917 Patented Aug, 7, T923.

WILLIAM WILSON, 0F EASTTJRANGE, NEW JERSEY, ASSIGNOR T0 WESTERN ELECTRIC COMPANY, INCORPORATED, 0]? NEW YORK, N. Y., A GORPORATION 015 NEW YORK.

THERMIUNICALLY-AQIIVE SUBSTANCE Application flied. July 26,

To all whom it may concern:

Be it known that I WILLIAM Winsome subject of the King of Great Britain residing at East Orange, in. the county of Essex and State of New Jersey, have invented certam new and useful Tm rovements inthe Thermionically-Active ubstances and Methods of Making the Same, of which the following is a full, clear, concise, and exact descri tion.

his invention relates to thermlonically active substances, which may serve as a source of electrons in a thermionic device, such as an audion and the method of making the same. It has been known for some time that platinum filaments coated With. alkaline earth oxide have a high order of thermionic activity. In case it is desired to operate a thermionic filament at a temperature higher than platinum will tand, it is necessary to use some other metal, such as tungsten, tantalum or molybdenum. These bare metals, however, have not a very high order of thermionic activity. An object of the present invention is to subject these metals to a special treatment whereby their thermionic activity is increased, and made comparable to that of oxlde-coated platinum filaments, and the advantage of being able to use high operating temperatures is retained. This same special treatment may be applied to other metals such as iron and nickel. While the use of the latter metals does not permit as high operating temperatures as in the case of tungsten, etc, still their use is advantageous because iron and nickel are comparatively cheap. Une well known way of maln'ng platinum-coated filaments is to make a paste with water of the carbonate or oxide, for instance, of one of the alkaline earth metals, that is, calcium, barium or strontium. A coating of the paste is applied to the filament to be treated and an electric current is passed through the coated filament to bake and fix the coat thereon. If some metal other than platinum, for instance, nickel, iron, tungsten or tantalum, is coated in thi way with an alkaline earth compound, the activity of such a filament will not be increased to the same extent as if treated by the method now to be described. In accordance with the present invention, it has been discovered that, if the metals such as nickel, iron, tungsten or tantalum AND METHOD OF MAKING- THE SAME.

1917. serial No. 182,965.

are cleaned by pre-glowing in vacuo or in theabsence of oxygen to a high temperature and then coated, the thermionic activity of the same will have been very greatly increased and will be about as thermionically active as coated platinum. In the pro-glowing, the pressures that have been used var from one-tenth to one-tenthousandth millimeter. The flashing temperature in each case should be raised as near as possible to the melting point. It is only necessary to maintain the filament at this temperature for a short time, viz, about five seconds. 7

After the filament has been pro-glowed, it is not essential that it be coated in the particular way outlined above. 15 or instance, the filament may be coated by sputtering the coating material on the filament. This is done by placing the filament near the coating material, both being in vacuo. The coating material is heated and it sputters and deposits on the hot filament.

When the filament is cleansed as indicated above by flashing in vacuo or in absence of oxygen, occluded gases are driven out or the filament and any oxides that are on the filament are reduced. Best results are obtained if the filament is kept in this condition, and to this end, after the filament has been cleansed in this manner, all subsequent heating of the filament would be in vacuo or in the absence of oxygen, to prevent re-oxidization and re-absorption of gases. lln certain cases it may be suficient to perform the-above process of flashing and baking in the absence of oxygen.

Apparatus that may be used in carrying out both the pre-glowing and the sputtering processes, is shown in the figure of the drawing in which a boat 1 of molybdenum, for instance, contains the coating oxide or material 2 with'which it is desired to coat the filament 3. This coating material may be a mixture of strontium and barium oxides for instance. The boat 1 and the filament 3 are sealed into a glass tube 4:, and may be heated electrically by current from the sources of current 5 and. 6, respectively. The glass tube 4: is provided with an out let 7' through which it may be evacuated.

During evacuation of the glass tube, the filament and the boat are heated to a high temperature to drive ofi occluded gases. When a suficiently high vacuum is attained,

the filament is flashed to a very high temerature, this prev-glowing serving to clean its surface thoroughly. The filament is then allowed to cool and the molybdenum boat heated to a high temperature (more than two thousand degrees 0.). Some of the coating material 2 sputters or evaporates and is condensed on thefilament,

which, as indicated above, may be, for ex-- ample, nickel, iron, tungsten or tantalum.

inall cases, it is preferred that after the pre-glowing or flashing in vacuo all subsequent heating of the filament should be in vacuo, as it has been determined that if the subsequent heating is in air, the advantage of pre-glowing is lost and the activity 1s diminished. If, instead of coating or sputtering, the filament is coated by the usual pasting process in the atmosphere, this coatmg may be permitted to dry in air at ordinary temperatures, but the coating should be baked on the filament in vacuo or in the absence of oxygen, and not in air.

What is claimed is:

The method of making a thermionic cathode which comprises highly heating a metallic filament in vacuum, sputtering on said filament a coating of alkaline earth oxide in vacuum, and baking the coated filament in vacuum.

In witness whereof,,I hereunto subscribe my name this 24th day of July A. D., 1917 WILLIAM W'IISON. 

